The total amount of production of asphalt mixtures is decreasing each year after reaching a peak in 1989 (see Non Patent Document 1). This is because the length of roads paved with asphalt in Japan has already reached 1,190,000 km and hence the number of roads to be newly constructed is reducing. Meanwhile, the reclamation rate of asphalt pavement waste, which was 7.6% in 1989, continues to increase abruptly, reaching 73.1% in 2008. This trend is expected to become stronger and stronger in the future as well.
Two methods each involving restoring the physical properties of an asphalt component that has deteriorated over time with an additive for reclaiming asphalt have been known for the reclamation of asphalt pavement waste (see Non Patent Document 2). One of the methods is a road-surface recycling method (remix system and repave system) but this is seldom adopted in Japan. The other is a plant recycling pavement method (plant mix system) which has gone mainstream in Japan. Several kinds of products (each using, for example, a semi-refined oil at the time of petroleum refining as a main raw material) have been commercially available as additives for reclaiming asphalt that can be used in the plant recycling pavement method. However, the products are not only expensive, but the products have high viscosities (are greasy) at room temperature and hence need to be heated to about 70° C. to 80° C. at all times. Accordingly, it is inconvenient to handle the products and heat source costs are required. In addition, concerns are raised about the effect of the products on the environment and humans because the products each contain large amounts of aromatic components.
In view of the foregoing, for example, an additive for reclaiming asphalt obtained by blending a mineral oil having a kinematic viscosity at 60° C. of 100 to 1,000 mm2/s, and a fat, the additive having a kinematic viscosity at 60° C. of 20 to 300 mm2/s and a flash point of 220° C. or more (see Patent Document 1), and an additive for reclaiming asphalt obtained by blending a mineral oil having a kinematic viscosity at 40° C. of 300 to 900 mm2/s and a polycyclic aromatic content of less than 3 wt %, and a fat, the additive having a kinematic viscosity at 40° C. of 40 to 400 mm2/s, a polycyclic aromatic content of less than 3 wt % and a flash point of 220° C. or more (see Patent Document 2), have been proposed. However, the high-viscosity mineral oils used in Patent Documents 1 and 2 are expensive because the oils are generally refined to a high degree, resulting in the prices of the products also become high.
In addition, modified asphalt obtained by adding plastic, rubber, or the like to straight asphalt to improve the properties of the asphalt has been known. For example, Patent Document 3 proposes a modified asphalt formed of asphalt, a waste plastic such as a waste polystyrene, and a waste oil containing an engine oil as a main component. An upper limit for the content of the waste plastic in the modified asphalt disclosed in Patent Document 3 is set to 40 wt %. This is because when the waste plastic is added at a content in excess of 40 wt %, curing starts immediately after the mixing of the materials and hence the resultant product cannot be used as a modified asphalt (see Comparative Example of Patent Document 3). In the industry, however, a modified asphalt blended with an additionally large amount of plastic, rubber, or the like is in demand in expectation of further improvements in the properties of asphalt.